Fashion designer Catherine Malandrino is suing former business partner Elie Tahari for $15 million, claiming he turned her high-end couture into a bargain-basement clothing line.

He also cut her out of the $7 million sale of her brand to the mass-market company Bluestar Alliance, the Manhattan Supreme Court lawsuit says.

Malandrino, in a 55-page filing, moans that the loss of her 28-year-old brand is “gnawing at her heart” and “crushing her vibrant spirit.”

The onetime “Project Runway” judge sold Tahari and his partner a 75 percent interest in her brand in 2011 following the global financial crisis, the suit says.

In exchange, she was paid around $5 million and given a five-year, $450,000 executive compensation deal, her lawyer, Richard Hott, said.

She believed Tahari would be able to help her expand her brand.

But then Tahari turned around and unloaded Malandrino “like a slave” to down-market Bluestar Alliance, which owns the teen line Hot Kiss, the suit says.

A spokesman for Tahari said, “We tried to help Catherine through her many financial challenges because we believed in her talent and her brand. We are surprised that she has chosen this course of action and sure that the courts will agree that the lawsuit is completely baseless and without merit.”